The extensive data storage needs of modem computer systems require large capacity mass data storage devices. A common storage device is the rotating magnetic hard disk drive.
A disk drive typically contains one or more smooth, flat disks which are rigidly attached to a common spindle. The disks are stacked on the spindle parallel to each other and spaced apart so that they do not touch. The disks and spindle are rotated in unison at a constant speed by a spindle motor.
Each disk is formed of a solid disk-shaped base or substrate, having a hole in the middle for the spindle. The substrate is commonly aluminum, although glass, ceramic, plastic or other materials are possible. The substrate is coated with a thin layer of magnetizable material, and may additionally be coated with a protective layer.
Data is recorded on the surfaces of the disks in the magnetizable layer. To do this, minute magnetized patterns representing the data are formed in the magnetizable layer. The data patterns are usually arranged in circular concentric tracks. Each track is further divided into a number of sectors. Each sector thus forms an arc, all the sectors of a track completing a circle.
A moveable actuator positions a transducer head adjacent the data on the surface to read or write data. The actuator may be likened to the tone arm of a phonograph player, and the head to the playing needle. There is one transducer head for each disk surface containing data. The actuator usually pivots about an axis to position the head. It typically includes a solid block near the axis having comb-like arms extending toward the disk, a set of thin suspensions attached to the arms, and an electro-magnetic motor on the opposite side of the axis. The transducer heads are attached to the suspensions, one head for each suspension. The actuator motor rotates the actuator to position the head over a desired data track. Once the head is positioned over the track, the constant rotation of the disk will eventually bring the desired sector adjacent the head, and the data can then be read or written.
Typically, a servo feedback system is used to position the actuator. Servo patterns identifying the data tracks are written on at least one disk surface. The transducer periodically reads the servo pattern to determine its current radial position, and the feedback system adjusts the position of the actuator as required. Older disk drive designs often employed a dedicated disk surface for servo patterns. Newer designs typically use embedded servo patterns, i.e., servo patterns are recorded at angularly spaced portions of each disk surface, the area between servo patterns being used for recording data. The servo pattern typically comprises a synchronization portion, a track identifying portion for identifying a track number, and a track centering portion for locating the centerline of the track.
The transducer head is an aerodynamically shaped block of material (usually ceramic) on which is mounted a magnetic read/write transducer. The block, or slider, flies above the surface of the disk at an extremely small distance as the disk rotates. The close proximity to the disk surface is critical in enabling the transducer to read from or write the data patterns in the magnetizable layer. Several different transducer designs are used, and in some cases the read transducer is separate from the write transducer.
As computer systems have become more powerful, faster, and more reliable, there has been a corresponding increase in demand for improved storage devices. A key constraint in the design of disk drives is the data density of the disk surface, i.e., the number of units of information that can be stored on a given unit of area of disk surface. In recent years, dramatic increases in data density have made it possible to increase the amount of data stored on disk drives and at the same time reduce the physical size of drives (which tends to reduce cost, increase speed, and lower the amount of power consumed). Continued progress in the art demands further increases in data density.
The maximum data density may be limited by numerous factors. For example: the magnetic characteristics of the disk surface may limit the capacity to record data; the smoothness of the disk surface and aerodynamic characteristics of the head may limit the proximity of the transducer to the surface, thus limiting its ability to read data from and write data to the disk; the amplification electronics may have limited sensitivity; or noise generated by other components may limit what can be read or written. But one of the key determinants of data density is the design of the transducer itself.
Older disk drive designs typically employed a single read/write "inductive" transducing element having an electrical conductor winding around a magnetically permeable material. Data was written by driving a current through the conductor coil to create a magnetic field, causing a residual magnetic pattern representing data to be placed on the disk surface. Data was read by passing the transducer over the disk surface and sensing an electric current induced in the conductor coil by the moving magnetic field. This type of transducer was used extensively for many years, and is still employed in some disk drive designs. However, as data densities and rotational velocities increase, it becomes increasingly difficult to read data using an inductive transducer. I.e., increasing density means that the magnetic patterns used to induce current in the transducer become smaller, while increased rotational velocities increase the frequency of the response, thus requiring faster and more sensitive read amplification electronics to detect the minute induced current. Additionally, smaller diameter disks have lower linear velocities, which reduce the output of an inductive read element.
As a result of the limitations inherent in inductive transducing elements, many disk drives now employ a magnetoresistive transducer design. Typically, a magnetoresistive transducer design employs a dual-element transducer having a conventional inductive element for writing data only, and a magnetoresistive element for reading data. The magnetoresistive read element exploits the fact that certain materials change electrical resistance in the presence of a magnetic field (which is known as the magnetoresistive effect). By passing a small bias current through such an element, the changes in resistance can be measured as the element passes over the tiny magnetic patterns on the disk surface.
The advantage of the magnetoresistive read element over the older inductive read element is that the magnetoresistive element is far more sensitive to small magnetic variations, enabling it to read more dense data patterns. However, such read elements have peculiar characteristics of their own, which must be taken into account if further improvements are to be made in data density.
The radial distance between the centerlines of adjacent tracks is referred to as the track pitch. When writing or reading data in a disk storage device, it is important to position the transducer accurately with respect to the track centerline. If the transducer is improperly positioned when writing data, data will be written off-center, making subsequent reads of the data difficult, and possibly corrupting data in an adjacent track. If the transducer is improperly positioned when reading data, the data read may be corrupted. The accuracy with which the servo feedback system can position the transducer ultimately affects the minimum possible track pitch and hence the data density.
A typical inductive write element is slightly narrower than the track pitch to avoid interference with adjacent tracks (approximately 80-90% of the track pitch). Typically, a magnetoresistive read element has an asymmetrical sensitivity. I.e., on one side of the element, the response of the element to a magnetic field tapers off more gradually than on the other side. In order to avoid noise that would result from reading magnetic patterns at the fringes of a track, the magnetoresistive read element is typically narrower still than the write element. Usually, the read element is approximately 60% as wide as the track pitch. Notwithstanding the narrowness of the read element, it is more than adequately sensitive to read the magnetic patterns on the disk surface, provided that it is centered over the track. However, the reduced read element width causes problems when reading servo patterns, particularly, servo patterns which locate the track centerline. The problem is not one of lack of sensitivity of the element, but one of simple geometry. A conventional servo pattern optimally reveals misalignment of the head with the track when the read element is approximately as wide as the fall track pitch, and thus able to span the full servo pattern. In the case of a magnetoresistive read element, this problem is aggravated by the asymmetrical response characteristic.
In response to this condition, it is known to use servo patterns having a finer resolution, thus enabling more accurate positioning with the narrower read element. While this works well in theory, it can be difficult and costly to manufacture the drive, and additional problems may be introduced.
An alternative approach is proposed in Japanese patent publication JA-4-205808 (1992). Using this approach, more than two leads are attached to a single magnetoresistive read element. A pair of leads separated by a distance d is used for reading data, while a different pair (which may or may not include a common lead) separated by a distance d', greater than d, is used for reading the servo pattern. In this manner, the width of magnetoresistive element used for the servo pattern can be wider than that used for data.
However, JA-4-205808 fails to recognize the distortion caused by additional leads in the magnetoresistive element. When reading a servo pattern, it is expected that only a portion of the element may be directly over the pattern. Proper interpretation of the position of the element requires the electronics to determine which percentage of the element is over the pattern, and for this purpose the element should ideally have a uniform response characteristic across its entire length. The leads which are used for reading data represent discontinuities in the response characteristic of the element when reading the servo pattern, making it difficult to accurately determine the position of the element with respect to the servo pattern.
It is desirable to further improve the tracking accuracy of a disk drive using magnetoresistive head technology without the aforementioned disadvantages.